The present invention relates to an optical signal transmission system and, more particularly, to an optical signal transmission system comprising an optical transmission line with at least one optical wave guide switch and an optical level adjuster.
Switching matrices are required in fiber optic systems, especially information transmission system, for switching between respective input and output wave guides. The optical cross-connect that combines the switching matrix with optical amplifiers for level regulation and/or transponders for wavelength conversion and/or WDM-components for wavelength separation is an especially important application for this type of switching circuit. In a complex cross-connect of this type level regulation means, which comprise a power measuring device arranged in the vicinity of the output optical fiber (by diverting a fixed part of the light power conducted in the output fiber to a photodetector) and one or more level adjusting devices, is an absolute necessity. The required quality, for example a guaranteed maximum error rate, of the data flow over several cascaded transmission paths and through several network junctions is only guaranteed by this type of level regulation in the larger photonic networks.
The level adjusting device should be controllable with electrical parameters (a voltage or a current level) and has at least one adjusting speed corresponding to the switching speed of the switching matrix. Its minimum insertion attenuation should be small, the adjusting range should be sufficient for compensation of different input levels and different insertion attenuation of the components in the switched paths, typically from 10 to 20 dB being required.
In the following the state of the art for the optical switch and then for the optical level adjuster or adjusting device is described.
Switches that can switch the path of light conducted in wave guide/glass fibers are required in fiber optic systems for special communication systems. This type of switch typically has one or more input wave guides and one or more output wave guides that are connected with each other by a switching matrix. The switching matrix causes a coupling of an input with a particular output wave guide (Proceedings ECIO""97, Stockholm, pp. 10/JWA4-1 to JWA4-6/15, Mats Gustavsson, et al: xe2x80x9cNetwork Requirements on Optical Switching Devicesxe2x80x9d).
Primarily it is desired that as much of the light from the input wave guide is coupled as completely as possible into the selected output wave guide (high transmission=reduced insertion attenuation) and all other output wave guides receive a greatly reduced proportion of the light in the input wave guide (reduced cross talk=high cross talk damping). That should be possible in the wide optical wave length range used for telecommunications purposes, typically from 1.2 to 1.6 xcexcm. Conventional specifications for an optical switching matrix used in telecommunications networks include insertion attenuation of less than 5 to less than 10 dB and cross talk attenuation of greater than 30 to greater than 60 dB.
Optical switches are embodied in a variety of technologies and have different switching principles.
Relay type switches shift a glass fiber in front of an array of opposing glass fibers so that a coupling between two fibers occurs with a precise alignment. Insertion attenuation and cross talk attenuation are outstanding, however the switching time is comparatively long and the manufacturing expense is great. The same goes for those switches that broaden the light from each glass fiber by collimated optics and then make a changeable optical coupling between different fibers with rotated or shifted mirrors, prisms and/or movable lenses.
Several switching principles are known for integrated optic circuits that operate entirely with wave guides.
Either the light of an input fiber is divided into several paths with a passive power divider and a switchable optical amplifier which has a high damping in the switched-out case and compensates for the loss in light distribution (active) in the switched-on case is inserted in each path. This amplifier together with a passive power divider can be manufactured in a semiconductor base (Semiconductor Laser Amplifier, SLA). The amplification process (based on induced emission) is generally a narrow bandwidth amplification and thus limited to a narrow wavelength range. Furthermore since the optical amplifier noise causes interference at high input powers, which is polarization dependent in its properties for the typical wave guide in a semiconductor substrate, the embodiments for the interesting wavelength range (1.2 to 1.6 xcexcm) must be constructed using the very expensive InP semiconductors.
In contrast a purely passive integrated optic wave guide switch operates by modification of the index of refraction distribution in the vicinity of the coupling points of wave wave guides. Either interferometric devices, for example a Mach-Zehnder interferometer (MZI) with two inputs and two outputs in which the coupling between inputs and outputs can be changed by changing the index of refraction (transit time, phase) in one or both arms, or X- or Y-branching devices with reduced opening angle, can be used for the switch so that the light is conducted into one of two outputs. The latter type of device has the advantage that the switching operation is guaranteed over a wide range of refraction changes (the so-called xe2x80x9cdigital optical switchxe2x80x9d, DOS), while the index of refraction must be very precisely changed in the MZI switch.
A temperature change can be used as a mechanism for index of refraction changes (Proc. 21st Eur. Conf. On Comm. (ECOC""95-Brussels, Th.L.3.5,pp. 1063 to 1066,R. Moosburger, et al: xe2x80x9cA novel polymer digital optical switch with high temperature stabilityxe2x80x9d). The wave guide material must have a high thermooptic coefficient (TO); typically organic materials (polymers, among others plastic materials) can be used. The switching time should be in the range of 1 ms, and the switching properties should be independent of light polarization. Instead of that the electrooptic effect (EO) which causes a index of refraction change under strong electric field in certain materials can be used (Y. Silberberg, et al, xe2x80x9cDigital Optical Switchxe2x80x9d, Appl. Phys. Lett. 51 (16) Oct. 19, 1987, pp. 1230 to 1232); here the switching time is very short (ps time scale), however the effect is different according to the light polarization. Furthermore materials with a high electrooptic effect, for example LiNbO3 are expensive.
Another switching principle is used in thermooptic digital optical switches (TO-DOS). According to FIG. 1 of the accompanying drawing wave guides 6 are in the polymer layers 3,4,5 provided in the silicon substrate 2. One known embodiment is the strip wave guide in a three layer system; however other optical configurations are conceivable. Heating conductors 7 that increase the temperature in the vicinity of the wave guides 6 and thus lower the index of index of refraction in that material are provided in the vicinity of the wave guides 6 over the wave guides 6, which means opposite to the silicon substrate 2 serving as heat sink. Typical index of refraction differences for the wave guide core-substrate are 5xc3x9710xe2x88x921, while the thermooptic coefficient is typically about 10xe2x88x924 Kxe2x88x921. Thus a temperature increase of 50 K in the wave guide core is sufficient to cut off the light conduction. A reduced temperature difference between both wave guide outlets of about 20 K is enough for switching in a gradually opening Y branch device. Thus the desired coupling over of the light into a particular one of the outputs is the more complete, the less the opening angle of the Y branching device is; because of that the component length of the wave guide switch increases. Since the plastic materials used have high thermooptic coefficients, however also even with reduced optical absorption, an increased component length leads to higher insertion attenuation. Furthermore the Y branch itself produces an additional attenuation in contrast to a simple straight or curved wave guide, since the distribution of the input wave guide section into the output section cannot be accomplished perfectly in the vicinity of the xe2x80x9cbranch pointxe2x80x9d of the xe2x80x9cYxe2x80x9d. The cross talk damping or attenuation attainable with this type of TO-DOS device is in a range of from 20 to 25 dB and is thus insufficient. Thus frequently two DOS switches are cascaded for a switching stage in order, because of that, to increase the cross talk attenuation so that it is greater than 40 dB. This procedure however clearly increases the insertion attenuation.
Besides the 1xc3x972-Y-TO-DOS switch also a 1xc3x973 switch is known according to the same principle. Larger switching matrices, for example 4xc3x974, 8xc3x978 or 1xc3x9716, can also be built by cascading these basic elements.
In a parallel patent application xe2x80x9cOptical Wave guide Switchesxe2x80x9d (Inventors: Winfried Bernard and Wolf-Henning Rech) an additional modulator for influencing the index of refraction is provided in an optical wave guide switch with an optical wave guide formed with a Y branch and devices arranged in it for causing local changes in the index of refraction in the output arms of the Y branch. This additional modulator is controlled so that the increased damping or attenuation obtained in one output arm by one of the devices is complemented by an increased modulation attenuation.
Of the integrated optical switches thermooptic switches best fit the specification of optical cross-connect systems.
Optical level adjusting devices chiefly comprise a glass substrate provided with vapor-deposited metallic reflection layers in which the reflection degree can be changed by variation of the layer thickness along a linear or circumferential coordinate of a rectangular or circular substrate. This xe2x80x9cgrey wedgexe2x80x9d is then inserted in the light path. By shifting or rotating the reflection degree can be changed and thus the remaining transmission can be changed. A precise electrical control is possible with a displacement sensor having an electrical drive means. In order to guarantee a reduction in the attenuation, the optical beam diameter must be considerably widened in the vicinity of the inserted attenuation plate in relation to that of the glass fiber so that additional lenses or lens-like media must be built in for beam transformation. All these components must be precisely adjusted. This type of level adjusting device is used as a precisely calibrated attenuation device, especially in light wave guide measuring technology, however it is too expensive for use in an optical cross-connect.
Another form for an optical level adjusting device comprises an integrated optical Mach-Zehnder interferometer which couples a portion of the light conducted through it into the wave guide substrate during tuning. Basically a thermooptically tunable symmetric or asymmetric Y branch device designed as a variable attenuation component would be suitable as a digital optical switch, when it is operated in an xe2x80x9canalogxe2x80x9d range in the center between ends of the switch characteristic curve. This characteristic curve is very steep and has hystereses in part that make it unsuitable for use as a level adjusting device.
Finally the decoupling of light can occur directly in the substrate without additional wave guide elements or structures, if the index of refraction gradient can be made large enough so that the substrate index of refraction can be adjusted in the optical field in the immediate vicinity of the wave guide core which is higher than the core index of refraction and, because of that, a xe2x80x9ctunnelingxe2x80x9d of a portion of the light guided into the substrate is possible. This type of xe2x80x9ccut-off modulatorxe2x80x9d, of course with electrooptic instead of thermooptic tuning, was described in A. Neyer, W. Sohler, Appl. Phys. Lett. 35, 256 to 258 (1979).
It is an object of the present invention to provide an optical signal transmission system having an optical wave guide switch and an optical level controller of the above-described type that is improved in relation to the prior art optical signal transmission systems.
The heart of the invention, i.e. its essential feature, is the combination of a thermooptically controlled wave guide switching matrix with a thermooptically controllable level adjusting device in a common integrated optic circuit.
The special advantages of the invention result from the fact that switching matrices and level adjusting devices are continuously jointly required in important application involving optical cross-connects, and that both structural elements are technologically compatible, when both structural elements are based on the especially desirable thermooptic control principle.
The realization of a common integrated optic circuit for a switching matrix (or partial switching matrix) and one or more level adjusting devices requires no additional manufacturing expense, since the process steps and their number and sequence are identical for both devices. The integration only requires a system of lithographic masks for making the wave guide and the heating conductor which provide both components in a series circuit in a common substrate.
The optical signal transmission system according to the invention benefits from the known advantages resulting from integration including a reduced component size that can be significant with large switching matrices with many inputs and outputs and reduced manufacturing costs since all process steps must be performed only once in the manufacturing of the device that result from integration. However it also has additional advantages, namely that the expensive, highly precise coupling of the glass fibers to the integrated optic circuit is necessary only once and only a single housing with the required electrical contacts is necessary. Furthermore the integration of the invention has an additional advantage, namely that the entire insertion attenuation of the series circuit comprising the switching matrix and the level adjusting device is reduced in the integrated structure in comparison to the same structure provided in two separate components connected by glass fibers presently used in the state of the art, since each coupling location between the glass fiber and the integrated optic circuit produces unavoidable transmission losses. Finally the reliability of the entire cross-connect is increased when it includes fewer components and connection locations.